In recent years, demand for utilization of a radio wave for mobile communication increases. In contrast, wireless communication interference in a cell phone, a wireless LAN (Local Area Network), or the like and a radio disturbance in a TV set, a radio set, or the like frequently occur by electromagnetic interference of radio interference in a frequency band of a radio wave used in mobile communication. Such radio interference is emitted from various electronic devices that operate as noise generation sources and is a factor which causes electromagnetic interference of a wireless communication device that exists in the neighborhood of the noise generation source or the electronic device itself that is the noise generation source generating the radio interference. For this reason, it is strongly required to suppress the emission of the radio interference generated by the electronic device in order to ensure the communication quality.
When a substrate on which an electronic component is mounted is designed and developed, the electromagnetic near-field is measured in order to identify the radio interference generation source which causes the electromagnetic failure. In the electromagnetic near-field measurement, the neighborhood on the surface of the substrate is scanned by using an electromagnetic field probe, a frequency characteristic in the electromagnetic field is sequentially obtained at each measurement point, and a point at which the noise intensity is high is detected.
In the general electromagnetic near-field measurement, a maximum value or an average value of the intensity of the radio interference is measured at a measurement frequency by using a spectrum analyzer. However, in many cases, the intensity of the radio interference varies over time. Accordingly, it is difficult to correctly evaluate the electromagnetic radio interference by using the above-mentioned evaluation method using a single parameter.
Accordingly, an amplitude probability distribution measurement is proposed as a method to evaluate the variation of the radio interference over time and an influence on communication. As shown in Equation (1), the amplitude probability distribution (APD) is a statistical parameter calculated by using a ratio of a time Ti in which an amplitude envelope line exceeds an amplitude value Ek specified in advance to a total measurement time T based on the time-series measurement data of the obtained radio interference amplitude values. Because the amplitude probability distribution indicates a relation between an occurrence frequency of noise and an amplitude intensity, it is useful as a measure to evaluate the radio interference to the digital noise. By measuring this amplitude probability distribution, the influence of the noise that varies over time can be grasped.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                APD          ⁡                      (                          E              k                        )                          =                                            ∑                              i                =                l                            h                        ⁢                                                  ⁢                                          T                i                            ⁡                              (                                  E                  k                                )                                              T                                    (        1        )            
Further, as described in patent literature 1, the amplitude probability distribution has a high correlation with a bit error rate before error correction in the communication evaluation. Accordingly, an influence of the radio interference on a digital wireless communication system existing in the neighborhood of the radio interference generation source can be estimated based on the amplitude probability distribution under a predetermined specific condition. Here, the specific condition is a condition in which an internal noise level of a measuring instrument is equal to an internal noise level of a receiver, a receiver uses a synchronous detection method, the communication bandwidth of a communication system is equal to a measurement bandwidth, and the like.
Further, in patent literature 2, there is described a spectrum analyzer which measures the amplitude probability distribution of the radio interference. Such spectrum analyzer with a function to measure the amplitude probability distribution or a measurement device with a function to measure a CCDF that is also an radio interference parameter similar to the amplitude probability distribution measures the amplitude intensity of the radio interference in the set measurement bandwidth and obtains the amplitude probability distribution or the CCDF by an arithmetic processing. The CCDF described above is the abbreviation for Complementary Cumulative Distribution Function.
Further, in patent literature 3, there is described an electromagnetic field distribution measurement method taking into account the variation of the measurement value over time which can measure the electromagnetic field distribution by the amplitude probability distribution.
By using the technology described in patent literature 1 to patent literature 3, the amplitude probability distribution of the electromagnetic radio interference can be measured with a high accuracy.
Here, in order to evaluate the influence of the electromagnetic radio interference on the communication system, it is necessary to compare with a desired communication performance value which gives an permissible limit value. As this communication performance value, for example, a desired bit error rate at a predetermined signal intensity that is determined in advance can be used. However, it takes much time for the measurement value of the bit error rate to become a reliable value. Therefore, it is difficult for a user to grasp the influence of the electromagnetic radio interference on the communication system immediately.
In patent literature 4, there is described a method in which the amplitude probability distribution of the predetermined electromagnetic radio interference and the bit error rate under the influence of the radio interference are measured in advance, the amplitude probability distribution and the bit error rate are associated with each other and stored, the amplitude probability distribution data similar to the amplitude probability distribution that is newly measured is searched for from a storage medium, and the influence on the communication system is grasped.